It doesn't stop you having great exclusives.
But the PS4 is in part selling on the basis that we know that teams like Naughty Dog, Santa Monica, Guerilla, Media Molecule, Japan Studio, Evolution, Bend (possibly), etc, are going to make games for it, and they all have a pedigree. People feel like those studios are going to churn out some great titles, and they know for a fact that they'll never get those titles elsewhere.
When you buy exclusivity from a 3rd party, I have no idea what may or may not be available on your platform long term.
It's not hard to see why Nintendo and Sony are lauded for their in house studios and why it matter so much to gamers that they have them.